1. Field of the Invention
The present invention is directed to an apparatus and method for suppressing reflections of an acoustic reception signal at a transmitting/receiving ultrasound transducer.
2. Description of the Prior Art
Investigations of factors contributing to the quality of ultrasound images have shown that the contrast between image regions with few echoes and image regions which are rich in echoes is reduced because, among other things, a mirror or ghost image of the primary image is superimposed on the ultrasound image. The mirror image arises due to multiple reflections or multiple scattering between the ultrasound transducer and an acoustic boundary layer in an examination region such as, for example, tissue to be examined. The multiple reflections arise because the acoustic characteristics of the transducer and the tissue (for example) are different. The echo signals arriving after a transmission pulse are partially reflected by the ultrasound transducer, and enter into the examination region as undesired, new transmission pulses, and in turn generate echo signals which the system incorporates into the overall image. An ideal transducer would not reflect the echo signals, but would absorb all of the acoustic reception energy which is incident thereon. The effect of multiple reflections would then not arise. Image disturbances due to multiple scatter within the tissue, and due to inhomogeneous distribution of the speed of sound therein, would merely remain.
Reflections at the transducer surface can be reduced by the use of one or more matching layers. Materials having an acoustic characteristic impedance lying between that of the tissue being examined and the transducer ceramic are employed for such matching layers. The matching layers usually have a thickness of one-quarter of the acoustic wavelength.
This technique yields an improvement compared to the case without such matching, however, the aforementioned multiple reflection effect is nonetheless not sufficient avoided.
A further possibility for minimizing multiple reflections is to arrange a damping layer between the transducer and the examination region. The reflections and the echo signals resulting therefrom must traverse this layer two times more often than the primary echo. The multiple reflections are therefore more highly attenuated relative to the primary echo, and are substantially suppressed depending on the attenuation value of the damping layer. A disadvantageous of this technique, however, is that energy is also removed from the primary echo. For compensating this unwanted attenuation of the primary echo, the transmission power can be boosted up to a limit determined by the permissible stress on the tissue. The maximum penetration depth, or depth of the examination region, however, is reduced in comparison to an unattenuated system, because the attenuation is not compensated for in the reception mode.
Another possibility, which has not as yet been developed into a practical embodiment is the employment of piezo materials having an acoustic characteristic impedance which lies closer to that of tissue than in the case of standard ceramics. For example, the ratio of the characteristic impedances of standard ceramic to tissue is approximately 20, while that of composite materials is approximately 10, and that of PVDF is 2.4. The reduction of reflections at the transducer surface achieved by this measure, however, is slight. A disadvantageous in the use of PDVF is that approximately eight times the transmission voltage is required to obtain the same output signal as in a ceramic transducer.
Another theoretical possibility is to employ a long, continuous, (i.e., ungraduated) matching layer. As of yet, however, this theoretical possibility has not been technically implemented. The combination of a PVDF or composite transducer with an ungraduated matching layer, for example, is also conceivable.